Premature Births Linked to Scarcity of Men

Increasing fathers' support during pregnancy may lower rates of premature babies and babies born too small, new research suggests.

When men are scarce in a community, women are more likely to give birth early and to give birth to newborns who weigh less than 5.5 pounds (2,500 grams), which is considered low birth-weight, the new study finds. Both prematurity and low birth-weight are linked to serious health problems, including problems breathing, bleeding in the brain and long-term cognitive problems.

The study doesn't definitively prove why men might be crucial to pregnancy health, but the reason could be evolutionary, said study researcher Daniel Kruger, a professor of health behavior and health education at the University of Michigan. When men invest little in a pregnancy, women may respond by investing less physically, too, unconsciously expecting the baby not to make it to adulthood, he said. It's a pattern that could have been set up early in the human species' evolutionary history, when infant mortality was high.

"I don't think any woman is consciously thinking, 'I should have a premature baby,'" Kruger said in a statement. "It is likely a non-conscious system regulated by hormones."

Linking fathers and babies

Previous research on the hunter-gatherer Aché tribe in Paraguay shows that children whose fathers are involved in all things family are more likely to make it to adulthood. At the same time, studies in industrialized North America find that in areas where men are scarce, marriage is less frequent, divorce is more common and more households are headed by single moms, than in industrialized places where the gender ratio is even. [The Sex Quiz: Myths, Taboos & Bizarre Facts]

Kruger and his colleagues were interested in finding out if these two seemingly disparate facts could be linked to the health of infants. About 1 in 12 babies born in the United States weighs less than 5 pounds, 8 ounces, the cutoff for low birth-weight, according to March of Dimes. (The average birth weight in the United States is about 7.5 pounds, or 3,389 grams.) About 1 in 9 babies are born before 37 weeks of gestation in the United States, compared with a norm of 40 weeks, according to the Centers for Disease Control and Prevention.

The researchers pulled together birth records from every county in the United States for the year 2000 from the CDC and compared the rates of low birth-weight and premature birth in each with Census data on the ratio of men to women in those counties. They also examined other demographic factors, including the economic status of those communities, the proportion of single moms and the educational attainment of residents.

The importance of fathers

The analysis revealed that male scarcity was linked to both low birth-weight and prematurity, with more of each in counties where men were well outnumbered by women. The research also replicated earlier findings showing that poorer and African- American mothers are more likely to have a premature or low birth-weight baby.

Still, the male scarcity effect remained even when the researchers accounted for socioeconomic status, race and education. Higher proportions of single motherhood partially explained the link, Kruger said, but that supports the evolutionary hypothesis.

"Single motherhood is very important, because it is a socio-demographic indicator of paternal investment — how likely fathers are to be with and support their children," he told LiveScience.

The idea that a deadbeat dad could influence a newborn's health may seem distressing, but the findings are hopeful from a public health perspective, Kruger said. If men's behavior changes infant health, then working with men to support their pregnant partners could have a major impact.

"If possible, we should increase men's involvement and support during pregnancy and reinforce the expectation that he will be around to help raise the child," Kruger said. "This does not just have to be financial support; it can be spending time with the child, taking care of him/her, teaching him/her skills, etc."

The researchers published their findings online Feb. 5 in the American Journal of Human Biology.

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Stephanie Pappas

Stephanie Pappas is a contributing writer for Live Science. She covers the world of human and animal behavior, as well as paleontology and other science topics. Stephanie has a Bachelor of Arts in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz. She has ducked under a glacier in Switzerland and poked hot lava with a stick in Hawaii. Stephanie hails from East Tennessee, the global center for salamander diversity. Follow Stephanie on Google+.